The invention relates to electromagnetic actuators for moving a valve in translation to bring it alternately into an open position and into a closed position. A major application lies in controlling the valves of an internal combustion engine, with spark ignition or compression ignition.
An electromagnetic actuator is known (U.S. Pat. No. 4,614,170) having an armature of ferromagnetic material driving the stem of a valve, resilient return means provided to hold the valve at rest in a middle position between its fully open position and its closed position, and electromagnetic means enabling the valve to be brought into both positions in alternation. The electromagnetic means described in document U.S. Pat. No. 4,614,170 has a first electromagnet with a ferromagnetic core placed on one side of the armature so that when excited it attracts the armature in a direction that tends to close the valve, and a second electromagnet placed on the other side of the armature so that when excited it tends to bring the valve into the fully open position.
Another electromagnetic actuator is described in U.S. patent application of Porcher et al. Ser. No. 09/806,711, claiming the priority of French patent application No. 98/12489. The electromagnetic means of that actuator have a single coil mounted on a ferromagnetic circuit of structure such that, in combination with the armature, it presents two stable magnetic flux paths both corresponding to small and generally zero values for the airgaps between the armature and the ferromagnetic circuit.
Those electromagnetic actuators are actuated as follows. The electromagnetic means enable forces to be exerted suitable for bringing the armature into a xe2x80x9chighxe2x80x9d position which is assumed to correspond to the valve being closed, and into a xe2x80x9clowxe2x80x9d position which corresponds to the valve being open, and enabling the armature to be held in both of these positions. In the xe2x80x9chighxe2x80x9d position the equipment compresses a spring for storing mechanical energy so long as a suitable current passes through the coil, or the single coil holds the armature. When the holding current is switched off, the spring propels the moving equipment towards its xe2x80x9clowxe2x80x9d position. A rod fixed to the armature pushes the stem of the valve and compresses the closure spring of the valve. At the end of the armature stroke, a holding current is established in the coil or a suitable coil to ensure that the valve remains open. The closure spring of the valve serves in turn to store energy and when the holding current is switched off it acts in turn to propel the valve and the armature upwards.
Some of the mechanical energy is lost to friction, impacts, eddy currents, and energy consumption due to counter-pressure forces, in particular on exhaust. Consequently, it is necessary to exert an additional or xe2x80x9cattractingxe2x80x9d force that is added to the force exerted by the springs in order to compensate for energy losses each time the armature goes from one extreme position to the other.
The additional energy supplied must be sufficient to guarantee that the armature travels a full stroke, but it must not be excessive so as to avoid terminal impact which would generate noise and wear. In practice, the impact velocity must not exceed a few hundredths of a meter per second (m/s) if noise and wear are to be maintained at acceptable levels.
Existing electromagnetic methods and apparatuses have difficulty in complying simultaneously with the above two conditions in simple manner. Either they must accept a high impact velocity, or else they require the presence of a position and/or velocity sensor which complicates the method and the apparatus and which increases the cost of implementation.
The present invention seeks in particular to provide a method and apparatus for electromagnetic actuation of a valve that provides satisfactory control of the amount of energy applied, but without requiring a sensor.
To do this, the invention makes use of the fact that the ferromagnetic circuit of the electromagnetic means can be made in such a manner as to ensure that an almost linear relationship exists between its reluctance R(x) and the size of its airgap x during the last fractions of a millimeter of the stroke before the armature sticks against the ferromagnetic circuit(s). This property is to be found in particular with single-coil electromagnetic means of the kind described in above-mentioned patent application No. 98/12489. In such an actuator, the inductance L(x) of the coil also varies in quasi-linear manner over a range beginning from immediately beyond the central position of the armature if the notches of the ferromagnetic circuit are of substantially the same length as the thickness of the armature. Since it is possible to calculate R(x) and L(x) on the basis of the current i passing through the coil (or two coils in series), it is therefore possible to calculate x at almost all instants after the central position has been passed, and thus to deduce velocity therefrom.
Consequently, the invention proposes in particular an electromagnetic valve actuator comprising a valve drive armature, resilient return means provided to hold the valve at rest in a determined position substantially halfway between two extreme positions including a valve closed position, electromagnetic means having a ferromagnetic core placed on both sides of the armature, and a power circuit for applying power in alternation to said electromagnetic means, the actuator being characterized in that the power circuit includes means for calculating the velocity with which the armature approaches each of its extreme positions on the basis of measuring the excitation current in the electromagnetic means and means for applying a current to the electromagnetic means in order to servo-control variation of said velocity to a determined reference profile without using a position and/or velocity sensor in addition to the driving coil(s).
The calculation means can deduce the variation in reluctance from the current measurement during the last part of the armature approach stage, i.e. for small airgaps, and it is possible to deduce the variation of x vs. time from the variations in reluctance.
In an advantageous embodiment of the invention, the calculation means are also designed to calculate repetitively the inductance of the electromagnetic means when the airgap exceeds a determined value, thus making it possible to determine corresponding values for x, e.g. by looking them up in a table. Under such conditions, the regulation can control a velocity variation profile over a major fraction of the stroke of the armature. In the range over which the relationships giving variation in L and R as a function of the airgap x are not very linear, an approximate value can be obtained for x at any instant by finding the center of gravity of the values of x as obtained by interpolating values of L and R as a function of x, on the assumption that the interpolation can be linear.
By means of this structure, genuine regulation can be performed as opposed to mere open loop control or as opposed to merely controlling the current delivered during a cycle on the basis of results obtained during a preceding cycle of armature oscillation.
The invention also provides a method of controlling a valve using such an actuator, in which the current passing through the electromagnetic means is sampled, variations in L(t) and R(t) are deduced from the current by calculation, then variations of x are divided by referring to stored tables, the residual velocity is derived from variations in x over time, and the application of a voltage to the electromagnetic means is controlled in such a manner as to servo-control variations in time of x to a predetermined profile.
In practice, the zone in which variation in L or R is not linear can be very narrow. R varies almost linearly as a function of x so long as the airgap x does not exceed a value x1 of about 0.5 millimeters (mm), for an actuator whose electromechanical portions have the structure shown in patent application No. 98/12489. Inductance L varies in quasi-linear manner as a function of x as soon as x exceeds a value x2 which is typically about 1 mm when the thickness of the armature is substantially equal to the thickness of notches formed in the magnetic circuit.
The characteristics of L and R as a function of x can be obtained for other types of magnetic circuit, derived from that described in application No. 98/12489.
The above characteristics, and others advantageously usable in connection with the above but capable of being used independently, will appear more clearly on reading the following description of particular embodiments given as non-limiting examples.